Infra-red flaw detector



Aug. 19, 1969 w. R. APPLE 3,462,602

INFM-RED FLAw ETEcToR Filed Au-g. 16, 1967 Difference AmDllfef Amplifier72 Alarm Trigger Marker Hff eso/:3. 76 30 '6 'i1/L fr@ @1M ATTORNEY.

United States Patent O 3,462,602 INFRA-RED FLAW DETECTOR Wayne R. Apple,Boulder, Colo., assignor to Automation Industries, Inc., El Segundo,Calif., a corporation of California Filed Aug. 16, 1967, Ser. No.661,022 Int. Cl. G01t 1/16; G01k 1/08; H013 39/00 U.S. Cl. Z50-83.3 7Claims ABSTRACT F THE DISCLOSURE The present invention relates to themanufacture of rolled steel stock, such as sheets and plates, and tomeans for insuring that the rolled stock is free from any internaldefects. This is accomplished by providing an infrared inspection systemwhich is effective to scan the stock while it is being rolled and/or itis still at an elevated temperature. The inspection system is effectiveto locate and identify discontinuities or defects, 'such as pipeinclusions and/or edge laminations by detecting variations in thesurface temperature of the rolled hot stock.

-Background of the invention In one steel manufacturing process a hotbloom or billet is rolled into thinner stock, such as a sheet or plate,hereinafter collectively referred to as rolled stock. The fully rolledstock may have a width on the order of up to 90 or 100 inches or moreand a length up to 30 feet or more. After being rolled to the requiredthickness, the stock is trimmed to the desired dimension, sorted andallowed to cool. Subsequently the stock may be fabricated into morecomplex structures, such as by cold rolling into thin sheets, cutting,stamping, bending, etc.

It has been found under some circumstances the stock may possess hiddendefects. In one type of defect the center portion of the stock separatesfor some reason, such as the presence of an inclusion. Since this makesthe stock hollow, this type of defect is normally referred to as a pipeinclusion. Under other circumstances an edge portion separates. Thistype of defect is commonly called an edge lamination. If the stockcontains defects of the foregoing variety, during the rolling operationsthe defect tends to grow in size and eventually cover a considerablearea. If this defect is not discovered during subsequent processing ofthe stock, a costly failure may occur.

Since defects of this nature are normally buried within the stock, it isvirtually impossible to detect the defects by a visual examination,particularlyat the speeds required in a modern rolling mill.Accordingly, numerous testing systems have been proposed to locate thedefects automatically. IFor example, it has been proposed to utilizeultrasonics, dye penetrants, magnetic, eddy current systems etc. Suchsystems have not been entirely satisfactory for several reasons. Amongother things, it has been necessary to allow the rolled stock to cool toambient temperatures before making the inspection. This, of course, it atime consumming operation. Moreover, this requires the inspection beingdelayed until after the completion of the rolling, final trimming andcropping operations. As a consequence, when a defect has been located ithas not been possible to salvage the rolled stock by judicious trimming,cropping, etc. Moreover, if additional rolling were required it wasnecessary to reheat the stock.

Summary of the invention The present invention provides means forovercoming the foregoing difficulties. More particularly the presice entinvention provides means for inspecting the stock while it is still hotand before it is trimmed or cropped. This is accomplished by providing arolling mill having an infrared inspection station therein whichreliably locates internal defects in the rolling stock by remotelysensing the elevated surface temperature of the rolled stock. Variationsin the surface temperatures beyond predetermined limits indicates adefect below the surface.

In the limited number of embodiments disclosed herein the infraredinspection station includes a pair of radiometers for scanning therolled stock at two different areas so as to be responsive to thedifference between radiations from these two areas. The two scanningareas are relatively close to each other whereby any naturally occurringtemperature changes will be substantially equal at both scan areas.However, if there are any defects, such as an edge lamination or pipeinclusion, there will be a corresponding variation in the differencebetween the temperatures and the two scan areas whereby the defect willbe reliably located.

Brief description of drawings These and other features and advantages ofthe present invention will become readily apparent from the followingdetailed description of a limited number of embodiments thereof,particularly when taken in connection with the accompanying drawingswherein like reference numerals refer to like parts and wherein;

FIGURE l is a block diagram of an inspection system embodying one formof the present invention;

FIGURE 2 is a fragmentary side view of a portion of the rolled stockbeing inspected by the system of FIGURE l;

FIGURE 3 is a side view of a portion of a rolling mill showing aninspection station embodying the inspection system of FIGURE 1;

FIGURE 4 is a block diagram similar to FIGURE l but showing aninspection station embodying a different form of the present invention,and

FIGURE 5 is a side view of an inspection station embodying another formof the present invention.

Description of preferred embodiments Referring to the drawings in moredetail and particularly to FIGURES 1 and 2, the present invention isparticularly adapted to be embodied in a system 10 for rolling bloomsinto steel sheets or plates. This system A10 includes a rolling station12, a trimming or cropping station 14, a rolling table 16 extendingtherebetween and an inspection station 18 disposed on said rolling table16 between the rolling station 12 and the cropping station 14.

The rolling station 12 includes a pair of enlarged rollers 20 adapted toreceive a billet or bloom from the blooming mill. These rollers 20 areeffective to compress or roll the billet or bloom into rolled stock 22having a substantially uniform thickness. The rollers 20' are normallyabout 2 to 3 feet in diameter and may be on the order of up to 5 or l0feet long whereby the finished stock 22 may have a corresponding width.

The lengths of the fully rolled stock 22 vary over a considerable rangebut they frequently have lengths on the order of up to about 20 or 30feet or longer. If the stock 22 is rolled to a thickness on the order ofabout 0.060 to about 0.25 it is normally referred to as a sheet.Whereas, if it is rolled to a thickness in a range from about 3756" upto about 1% or more, it is usually referred to as plate. However,throughout this application the expression rolled stock shall be used toindicate all types of rolled material without regard to the thickness.

Prior to, during and following the rolling operation the rolled stockhas an elevated temperature which, under some circumstances, may be inthe red hot region. The stock 22 is normally hot worked during therolling operation and the surface temperature remains in an elevatedregion. In a typical rolling mill the temperature of the fully rolledstock 22 as it leaves the last set of rolls is frequently in the regionof about 1800 (Fahrenheit). However, in some mills it may be in a regionextending from below l000 (Fahrenheit) to about 2000 (Fahrenheit) orhigher.

It' the original ingot is cast with any inclusions such as slag, clay,sand, air bubbles etc., the inclusions tend to remain in that portion ofthe ingot which is still in the liquid phase. Since the center of theingot is the last to solidify many of the inclusions are at or near thecenter of the ingot. As a consequence as the bloom is formed from theingot and the bloom is rolled into the stock 22, the inclusions tend toremain concealed below the surface. They are, therefore, extremelydifficult if not impossible to visually observe. Typically in the fullyrolled stock 22 the inclusions are normally centered approximatelymidway between the top and bottom surfaces of the stock 22, as seen inFIGURE 3. Moreover, as best seen in FIGURE l, the inclusions 24 arefrequently located near an end of the stock in the region of thelongitudinal centerline of the rolled stock 22.

During rolling the thickness of the stock 22 is decreased and thematerial spread over a wider area. At the same time any inclusions 24are also rolled flat and spread. As a consequence an inclusion of thistype may expand to cover several square feet by the time the stock 22 isfully rolled. Since this tends to produce a hollow region in the stock22 this type of defect is frequently called a pipe inclusion.

Under some circustances a small crack or inclusion may be present in thestock 22 immediately adjacent to the edge 28 thereof. This may cause theedge of the stock 22 to begin developing a laminar separation during thesuccessive rolling operation. Since these separations are normally at ornear the edge 28 they are frequently referred to as edge laminations 26.During the repeated rolling and working this separation or edgelamination 26 gradually grows inwardly from the edge 28 of the plate andmay also eventually cover up to several square feet, Edge laminationsmay be at or near one end of the plate, as at 26A, or near the middle,as at 26B.

The rolling table 16 is disposed adjacent to the rollers and receivesthe fully rolled stock 22 as it emerges from the rollers 20. This table16 includes a plurality of substantially parallel and horizontal siderails 30 with several relatively small diameter rollers 32 extendingtherebetween. These rollers 32 are adapted to support the weight of thestock 22 and allow it to be moved longitudinally on the table. Normallyat least a portion of the rollers 32 are power driven whereby theoperator can control the longitudinal position of the stock 22 on thetable 16 and can even cause it to be repeatedly passed back and forththrough the rollers 20 until it is rolled down to the desired thickness.

The cropping or trimming station 14 is disposed adjacent the end of therolling table 16. After the stock 22 has been passed through the rollers20 and reduced in thickness to the desired level, the rollers 32 aredriven to carry the stock 22 through the trimming station 14. At thispoint the ends and/or edges 28 of the stock 22 are trimmed or cropped toreduce the stock 22 down to the desired width and/or length. Also, ifthere are any defects present in the stock 22 the defective portions maybe cropped. This removes the defect and leaves entirely sound rolledstock. Since the stock 22 is still very hot at this time these trimmingor cropping operations may be easily performed. Following the trimmingor cropping operation the stock 22 may be sorted according to itsintended future use and/or shunted into a storage area and allowed tocool to ambient temperature, etc.

In order to locate the various defects, such as pipe inclusions 24, edgelaminations 26, etc. the inspection station 18 may scan the plate 22 asit travels across the rolling table 16, This inspection may occurbetween successive rolling operations. However, in the present instanceit is made after the rolling is completed and the stock 22 is ready tobe transferred to the cropping station 14.

The present inspection station 18 is of the so-called infrared varietywherein the radiations from the stock 22 are monitored. As a result thestation 18 may be effectively separated by a considerable distance fromthe stock 22 and thereby protected from extremely high surfacetemperatures on the rolled stock 22.

The station 18 includes radiation IR sensing means 34 for receiving theinfrared radiations and producing an electrical signal correspondingthereto. In the present instance the radiation sensing means iseffective to sense the radiations at two separate and distinct areas.This may be accomplished by a single pickup which alternately scans thetwo separated areas. However, in this embodiment two separate pickupsare provided for continuously scanning the two areas. Each pickupincludes a device, such as a radiometer 36 and 38.

Each of the radiometers 36 and 38 includes an optical head 40 and42having an infrared cell or similar device. The cell is disposed insidethe head so as to be responsive to the radiations in the wavelengthsnaturally radiated from the surface 44 because of its elevatedtemperatures. Suitable electronics are coupled to each of the cellswhereby electrical signals are provided having amplitudes that arefunctions of the intensity of the received radiations.

Lens system 46 and 48 are provided for the optical head 40 and 42. Eachlens system 46 and 48 is focused onto a relatively small scan spot 50and 52 respectively, on the surface 44. The radiations from these spots50 and 52 are concentrated into the respective cell. It may thus be seenthe signals produced by the radiometers 36 and 38 are functions of thesurface temperatures at the respective scan spots 50 and 52 and thedifference between the two signals corresponds to the difference betweenthe two temperatures.

The lens systems 46 and 48 are arranged such that the two scan spots 50and 52 are spaced a predetermined distance from each other. Although thedirection and amount of spacing can be varied to satisfy any particularrequirements, in this embodiment the scan Spots 50 and 52 are disposedlaterally of the rolled stock with one of the scan spots 50 disposednear the center of the stock 22. As the stock 22 travels across therolling table 16 the scan spot 50 follows a scan line 54 which extendsthrough the region where a pipe inclusion is normally most apt toappear.

The second scan spot 52 is laterally displaced from the first spot 50and will thereby follow a second sean line 56. This seocnd scan line 56is preferbaly displaced from the region where the pipe inclusions 24 aremost common. It will be seen if a defect, such as the pipe inclusion 24passes through the inspection station 18, one radiometer 36 will receiveradiatins corresponding to the temperature of the surface 44 adjacent tothe defect while the other radiometer 38 receives radiationscorresponding to the surface temperature of a defect free region.

The outputs from the two radiometers 36 and 38 are coupled to a suitableelectronic system for processing the temperature signals. In the presentinstance this includes a differential amplifier 64 having two inputs 58and 60 and a single output 62. The two inputs S8 and 60 are coupled tothe outputs of the two radiometers 36 and 38 and receive the temperaturesignals therefrom. The signal present on the output 62 will be afunction of the difference between the two signals. More particularly,if the ternperatures of the two scan spots 50 and 52 are identical thetwo temperature signals will be equal and the output will be zero.However, if one of the scan spots is hotter or cooler than the otherscan spot, a temperature differential exists and accordingly there willbe a corresponding difference signal present on the output 62. Theamplitude of the difference signal is a function of the difference-between the temperature of the two scan spots 50 and 52.

The output 62 of the difference amplifier 64 is coupled to an amplifier66 which amplies the difference signal to a more useful level andimproves the signalto-noise ratio. This amplifier is in turn coupled tosuitable output means for utilizing the difference signal. For example,the output means may include a meter 68 0r similar device to indicatethe difference :between the temperature of the scan spots.

In addition, the output means may include an automatic device such as areject or trigger circuit 70 which becomes hoperative when thetemperature differential is outside of a predetermined range. Forreasons that will be explained subsequently when an excessivetemperature differential does exist, a defective area is present. Thetrigger 70 may be coupled to an alarm 72 and/ or marker 74 forindicating to the operator the presence of a defective area and itslocation. As a result these defective areas can be removed at thecropping or trimming station 14.

As the stock 22 is rolled to its final dimensions and carried across thetable 16 it begins to cool. Because of convection cooling and for otherreasons the naturally occurring heat losses are approximately twice asgreat from the top surface as from the bottom surface. As a result thereis a general tendency for the heat to llow vertically through the stockwith the top surface 44 normally being considerably colder than thebottom Surface 76.

It can be appreciated if there are any internal discontinuities withinthe stock 22 there will be corresponding variations in the thermalconductivity and the rate at which the energy flows upwardly. Forexample, if there is an air pocket or void present, the tiow of heatfrom the bottom 76 to the top 44 will Ibe reduced. This producescorresponding localized variations in the temperature of the surface 44.As the scan spots '50 and 4S2 at the foci of the radiometers 36 and 38move along the scan lines l54 and 56, they will produce fluctuations inthe temperature signals. These fluctuations correspond to the localizedvariations in the surface temperatures.

The temperature differences occurring around defects tend to be ofrelatively small magnitude if the stock 22 is merely allowed to coolnaturally. As a consequence the radiometers 36 and 38 must be verysensitive to detect these variations and produce a signal having asatisfactory signal-to-noise ratio. It has also been found variations inemissivty etc. can produce variations that are significant compared tothe temperature changes produced by naturally cooling defects. Thetemperature variations can 'be increased if the rate of cooling isincreased and particularly if the increased cooling occurs on only oneside of the stock 22.

If the amount of cooling is of sufficient magnitude there will be a verylarge difference between the temperatures on the top and bottom surfaces`44 and 76. By cooling the underside of the stock the thermal energywill flow downwardly from a region just below the top surface 44 towardthe bottom surface 76. As a consequence the ilow of thermal energy will-be substantially entirely downwardly. If the rolled stock 22 has auniform thermal conductivity the temperature of the top surface 44 willbe uniform. However, if there is a discontinuity such as the inclusion24 or the edge laminations 26 there will be a correspondingdiscontinuity in the thermal conductivity. This, in turn, will result inthe temperature of the upper surface 40 directly over the defect coolingat a considerably slower rate than normal, i.e. it will Ibe relativelyhotter.

In order to produce the foregoing type of accelerated cooling, a coolingdevice such as a jet 7S may be provided below the rolled stock 22 in theregion where it leaves the rollers 20. This jet 78 directs a stream [t0`of coolant, such as cold air, water, etc., against the bottom 76 of thestock 22. This coolant absorbs large quantities of thermal energy andlowers the temperature of the bottom surface 76.

In a practical application it is normally preferable t0 direct a fairlylarge stream 80 of Water at about room temperature against the undersideof the stock 22. This stream 80 is directed against a substantial areathat extends across the width of the stock and is aligned with theregions containing the types of defects which are of interest. Forexample, if pipe inclusions 24 are of primary interest the stream 80covers the central portion of the stock 22. If edge laminations 26 areof primary interest the stream 80 covers the edge portions of the stock.If both types of defects are of interest both portions of the stock arecovered =by the stream. In addition, for reasons that will becomeapparent subsequently, the stream 80 also covers an area which isgenerally free of the foregoing types of defects.

The spots 50 and 52 are spaced a considerable distance, for exampleseveral feet, away from the area 82 cooled by the stream 80. By the timea particular part of the rolled stock 22 has traveled over the jet 78and reaches the scan spots 50 and 52, a large quantity of thermal energywill have flowed downwardly toward the -bottom surface 76 and thetemperature of the top surface will have been greatly reduced. The twoscan spots 50 and 52 are laterally spaced but both of the scan lines 54and -56 pass over the cooled region 82. The radiometers 36 and 38 willthereby produce signals that are functions of the reduced temperatures.

The temperatures at the scan spots 56 and 52 will be a function ofseveral factors, such as the initial temperature of the stock, its rateof travel, the amount of heat absorbed by the coolant, the time delaybetween the cooling and the scanning etc. In addition the temperaturesof the scan spots 50 and 52 are a function of the thermal conductivitiesof the rolled stock 22 underlying the scan spots 50 and 52. If the stock22 is uniform and free of defects, the conductivity is substantiallyuniform and the temperatuers of the scan spots 50 and 52 will besubstantially identical. However, if a defect is present as pointed outabove, the heat ilow is reduced and the surface temperature above thedefect is greater than normal, i.e. a hot spot is present. Under thesecircumstances a large temperature differential will be present betweenthe two scan spots 50 and 52.

It might be expected the two temperatures, the resultant infraredradiations and the signals from the radiometer would be substantiallyconstant (assuming there are no defects present). However, it has beenfound as a practical matter the stock 22 is frequently unevenly heated,there are irregularities in the rolling process, the speed of the stockvaries, the amount of heat absorbed by the coolant varies, the emissivtyvaries, etc. These factors result in significant variations in thetemperature of the surface. However, these variations occur primarilyinthe longitudinal direction over extended distances. The changes whichoccur in directions transverse of the stock are relatively small. As aconsequence although the temperatures and/or the radiations from the twolaterally spaced scan spots 50 and 52 may vary, they both vary insimilar manners whereby the temperature differential is relatively small(assuming there are no defects present). Moreover, if there is a defectpresent under only one of the scan spots, the temperature differentialwill be very large compared to the slowly varying factors. Accordingly,the reject level for the trigger can be set above the naturallyoccurring variations and below the defect produced variations.

In order to utilize this system the unrolled material is fed between thetwo rollers and onto the table 16. As the rolled stock 22 enters theinspection station 18 and jet 78 directs a stream 80 of coolant againstthe underside of the hot stock 22. This absorbs large quantities ofthermal energy from the bottom surface 76 and causes substantialquantities of the thermal energy to ow downwardly from the region of theupper surface 44. This, in turn, will cause the top surface 44 to coolat an accelerated rate. The two radiometers 36 and 38 will receive theradiations resulting from the temperatures of the two scan spots 50 and52 and will produce signals which correspond to these two temperatures.

If the rolled stock 22 has substantially uniform characteristics overits entire width and is free from any discontinuities such as the pipeinclusions 24 etc., the two scan spots 50 and 52 will have substantiallyidentical temperatures. These temperatures will depend upon variouscharacteristics, such as the initial temperature of the rolled stock,the amount of heat absorbed by the cooling jet, the rate of travel ofthe stock across the rolling table, the radiating characteristic of thesurface, etc. Normally all of these factors vary at a relatively slowrate. As a consequence if the two scan spots 50 and 52 are fairly closetogether, for example a few feet apart, the temperatures and theradiations will be substantially identical. The two temperature signalsare in turn coupled through the difference amplifier 64 whereby anamplifier signal is provided that is a function of the differencebetween the two temperatures at the two scan spots 50 and 52.

If there are no discontinuities aligned with the scan spots thetemperature differential will be zero or very small, i.e. less than thethreshold level of the trigger 70. As a consequence no defects will beindicated.

However, if there is a discontinuity, such as the pipe inclusion 24 oredge laminations 26, there will be a corresponding localized variationin the thermal conductivity through the thickness of the stock 22.Normally this produces a decrease in the conductivity, a correspondingdecrease in the rate of heat flow toward the bottom and an elevatedtemperature on the top surface immediately adjacent to thediscontinuity. Under these circumstances as the scan spot 50 travelsover the hot spot the temperature signal from the radiometer 36increases. Normally the second scan spot 52 is laterally displaced fromthe discontinuity and will continue to travel in a cooled region. Thedifference amplifier will now produce a large signal. The amplitude ofthis signal normally far exceeds the reject level and as a consequencean indication or alarm will be produced.

It can be appreciated that the present embodiment is primarily adaptedfor locating pipe inclusions disposed somewhere around the center lineof the stock. In the event it is desired to identify edge laminationsone of the radiometers may be positioned to place the scan spots nearthe edge of the stock whereby it will travel over any edge lamination.If it is desired to identify pipe inclusions and/or edge laminations onboth sides of the plate, more than two radiometers may be provided forproducing scan spots which are located at strategic areas on the stock.Moreover, a single radiometer may be provided for scanning laterallyacross the stock 22 and then gating the signal to provide signalscorresponding to the lateral areas of the stock 22.

Although the foregoing arrangement is effective it has been founddesirable, under some circumstances, to .utilize the embodiment 90 ofFIGURE 4. This embodi ment 90 is substantially identical to thepreceding embodiment in that it is disposed over the rolling table 16between the rolling station 12 `and the trimming or cropping station 14.In addition a cooling jet is disposed beneath the rolling table 16 so asto direct a stream of coolant against the underside of the rolled stock.This jet absorbs large quantities of thermal energy and cools the underside of the stock whereby a cooled region 92 is formed substantially thesame as in the preceding embodiment.

A pair of radiometers 94 and 96 are provided and focused upon the rolledstock so as to define scan spots 98 and 100 in substantially the samemanner as described in the preceding embodiment. However, in thisembodiment the two scan spots 98 and 100 are disposed in substantiallylongitudinal alignment with each other whereby both radiometers willfollow the same scan line 102 and scan substantially identicalmaterials. The two scan spots 98 land 100 are disposed on opposite sidesof the cooled region 92. Thus the two radiometers will now producesignals which-represents the difference between the temperatures beforeand after cooling.

If the stock 22 is sound and free from any discontinuities the topsurface will cool at a rapid rate. Accordingly, the temperaturedifference will normally exceed some predetermined level. However, ifthere is a discontinuity there will bea decreased rate of heat transferand the surface temperature will not cool as fast as normal. Thisresults in a temperature between the scan spots and 100 which is smallerthan normal.

The two radiometers 94 and 96 are in turn coupled to a differenceamplifier`104, an amplifier 106 and a trigger circuit 108. Thisembodiment functions essentially the same as the preceding system exceptthe trigger 108 now causes an alarm when the temperature difference istoo small.

As an alternative the embodiment of FIGURE 5 may be employed. Thisembodiment 116 is substantially the same as the two precedingembodiments in that it is normally located between the rolling station12 and trimming or cropping stations 14. Also the rolled stock 22 iscooled by a jet 118 which directs a stream 120 of coolant against thebottom surface 122 of the stock 22. In this embodiment a pair ofradiometers 124 and 126 are provided. However, they are positioned onthe opposite sides of the stock so as to form a scan spot on the uppersurface 128 and a scan spot on the bottom surface 122.

Normally these two scan spots are in direct alignment with each other.Thus the first radiometer 124 will produce a temperature signalCorresponding to the top surface temperature while the second radiometer126 will produce a signal corresponding to the bottom temperature. Thetwo radiometers 124 and 126 are coupled to a difference circuit foractivating an alarm when the temperature difference exceeds apredetermined level.

It can be appreciated if the stock 22 is of uniform acceptable qualityand free from any discontinuities, the temperature difference will bebelow a predetermined level. However, if there is a discontinuitypresent so as to impede the fiow of thermal energy between the twosurfaces 122 and 128 the temperature difference will exceed apredetermined level whereby the alarm will be actuated.

While only a limited number of embodiments of the present invention havebeen disclosed herein it will be readily apparent to persons skilled inthe art that numerous changes and modifications may be made withoutdeparting from the invention. Accordingly, the foregoing drawings anddescription thereof are for illustrative purposes only and do not in anyway limit the scope of the invention which is defined only by the claimswhich follow.

I claim:

1. A nondestructive tester for inspecting rolled stock following arolling operation, said tester including the combination of coolingmeans for reducing the temperature of at least a portion of said rolledstock whereby said portion is allowed to change its temperature for apredetermined time interval;

a pair of radiometers for receiving infrared radiation from the surfaceof said rolled stock and providing signals corresponding to thetemperatures of said surface,

focusing means for focusing said radiometers onto a pair of separatedscan spots whereby said signals are functions of the temperatures atsaid scan spots, said scan spots being disposed at two different pointswhich are cooled before being scanned by the radiorneters;

circuit means coupled to said radiometers and responsive to said signalsto produce a difference signal that is a function of the differencebetween the temperatures of said surface at two separate locationscorresponding to dilerences in the rate of cooling; and

utilizing means coupled to said circuit means and responsive to saiddifference signal, said utilizing means being effective to perform anoperation whenever the difference signal Varies beyond predeterminedlimits.

2. A nondestructive tester for inspecting rolled stock following arolling operation and while said stock is cooling, said tester includingthe combination of temperature sensor means adapted to be spaced fromthe stock for scanning said stock along at least one scan line, and

circuit means coupled to said sensor means being effective to produce anelectrical signal that is a function of the difference between thetemperatures of said surface lat two separate locations.

3. The nondestructive tester of claim 2 including utilizing meanscoupled to said circuit means and responsive to said electrical signal,said utilizing means beng effective to perform an operation whenever theelectrical signal varies beyond predetermined limits.

4. The nondestructive tester of claim I1 wherein said means includes ajet for directing a stream of coolant against a surface of theworkpiece.

5. The nondestructive tester of claim 2 wherein the temperature sensorincludes a pair of radiometers for receiving infrared radiations fromthe surface of the cooling rolled stock and providing signalscorresponding to the temperatures of said surface, and

focusing means for focusing said radiometers onto a pair of separatedscan spots whereby said signals are functions of the temperatures atsaid scan spots.

6. The nondestructive tester of claim 5 wherein the scan spots definedby said radiometers are disposed longitudinally of the workpiece on theopposite sides of the cooled portion, one of said scan spots beingpositioned before the area cooled by the jet and the other beingpositioned after said area whereby the difference signal corresponds tothe amount of cooling produced by the jet.

7. The nondestructive tester of claim 5 wherein the radiometers and thescan spots are disposed on the opposite sides of the workpiece wherebysaid signal corresponds to the difference between the temperatures onthe opposite surfaces of the workpiece.

References Cited UNITED STATES PATENTS 3,044,297 7/ 1962 Hanken.

3,188,256 6/1965 Shoemaker.

3,216,241 11/1965 Hansen.

3,245,261 4/1966 Buteuy et al.

3,295,842 1/ 1967 Stelling et al 73-351 3,206,603 9/ 1965 Mauro.

RALPH G. NILSON, Primary Examiner MORTON J. FROME, Assistant ExaminerU.S. C1. X.R. 73--351

